The question of the wettability of a surface by water forms the basis for a large number of technical solutions in the areas of seals, self-cleaning surfaces and drinking water production. Nature has already developed various solutions for this purpose, which are frequently the result of a structuring or a selected chemical composition of the natural surface.
One of the most well-known examples of this is the lotus plant, whose leaves, according to W. Barthlott and C. Neinhuis, Purity of the sacred lotus, or escape from contamination in biological surfaces, Planta 202, 1 (1997), are studded with micrometer-sized papillae, on which there is an epicuticular wax film. Thanks to a suitable combination of structuring and surface chemistry, the leaves of the lotus plant have a low wettability with water, which is the basis of the well-known self-cleaning ability of the lotus plant.
In their Fabrication of Superhydrophobic Surfaces with High and Low Adhesion Inspired from Rose Petal, Langmuir 26(11), 8207 (2010), B. Bhushan and E. K. Her describe a contrary effect in some varieties of rose. These roses have developed a surface on their blossoms, which can store a large amount of water. Although, similarly to those of the lotus plant, these surfaces are coated with a fine wax film, the different arrangement of the underlying microstructure, however, results in the water droplets effectively adhering to the rose petals.
According to A. R. Parker and C. R. Lawrence, Water capture by a desert beetle, Nature 414, 33 (2001), the Onymacris unguicularis beetle takes advantage of its wettability with water to an exceptional extent. In order to be able to take in sufficient amounts of water in the hot desert, it spreads out its wings, which are covered with micrometer-sized hydrophilic elevations and hydrophobic hollows, into the moist surroundings as the mist rises, whereupon the mist condenses on its wings and forms droplets, which, once they have reached a certain size, run directly into the beetle's mouth.
In order technically to manufacture superhydrophobic surfaces, J. Bekesi, J. J. J. Kaakkunen, W. Michaeli, F. Klaiber, M. Schoengart, J. Ihlemann and P. Simon, Fast fabrication of super-hydrophobic surfaces on polypropylene by replication of short-pulse laser structured molds, Appl. Phys. A 99, 691 (2010), utilize an injection molding technique, for which regularly arranged structures are used, the preparation of which requires the creation of a special, expensive mold insert. The elaborate production of a new mold insert is necessary in order to modify the structures or their function. In order to create undercuts, the mold inserts must then be reworked in an additional process step, in particular, using laser ablation. Ultimately, the increase in the contact angle achieved by this means is slight.
DE 10 2008 053 619 A1 discloses a method for the production of a technical molded body. For this purpose, a layer of plastic, which is able to be cured, is introduced between two parallel, thermally conductive plates, pressure is applied between the two panels and both panels are heated so that the layer is heated to a temperature above the glass transition temperature of the plastic. The two plates differ insofar as they have a different adhesive force or degree of roughness on their surface or are heated to two different temperatures. Both plates are then pulled apart while the temperature is maintained, whereby the layer remains adhered to one of the two plates, while an initial large number of filaments forms on the other plate. After the plastic has cured, a substrate is obtained, onto which a large number of initial filaments has been applied, and introduced between two parallel, thermally conductive plates. After pressure is applied and the two plates are heated to a temperature above the glass transition temperature, the two plates are pulled apart while the temperature is maintained, whereby the substrate remains adhered to one of the two plates, while a second large number of filaments forms on the other plate, the underside of which, in each case, is firmly bonded with the surface of each initial filament.
According to U. Fischer et al., Tabellenbuch Metall (Book of Metal Tables), 41st revised and amplified edition, Europa-Lehrmittel publishers, Haan-Gruiten (2002), the term roughness describes a form of the surface deviation, caused by deviations in the actual surface (metrologically recordable surface) from the geometrically ideal surface, the nominal form of which can be defined by a drawing. This type of unevenness of a surface height is metrologically recorded and is numerically characterized by means of the parameters of the mean roughness Ra and the averaged roughness depth Rz. According to W. Beitz and K.-H. Grote, Dubbel Taschenbuch fuer den Maschinenbau (Dubbel Handbook of Engineering), 19th edition, Springer-Verlag Berlin (1997), these roughness parameters are recorded based on the reference surface, which has, as a rule, the shape of the geometric surface and, in terms of its position in space, coincides with the principal direction of the actual surface.
The mean roughness parameter Ra corresponds to the arithmetic mean of the deviation of a measuring point on the surface from the center line. The mean roughness depth parameter Rz is determined by dividing a defined measurement section on the surface of the work piece into seven equally sized individual measurement sections, wherein the evaluation is only made over five of these sections. The difference from the maximum and minimum value is calculated for each of the individual measurement sections and the mean value is then derived from this.
The separation of oil/water emulsions is a major technical challenge. The removal of layers of oil, particularly due to environmental catastrophes on the open sea, is brought into focus again and again.
Essentially, there are three options for the removal of a layer of oil at sea: skimming the oil, accelerating the natural decomposition of the oil by adding dispersants, and incinerating the oil film on the open sea. Adequate disposal of the oil is not possible using the two latter options; skimming the oil is prioritized in Juuso T. Korhonen, Marjo Kettunen, Robin H. A. Ras and Olli Ikkala, Hydrophobic nanocellulose aero-gels as floating, sustainable, reusable, and recyclable oil absorbents, ACS Appl. Mater. Interfaces, 3 (6): 1813-1816, 2011. Selective sorbents must be utilized In order to avoid the elaborate processing of the skimmed oil/water mixture. Generally, natural absorbents are called on for this purpose. For example, sawdust, rice straw or cotton are used as natural absorbents. Indeed, these absorb oil, although in very small amounts. Their capability, however, is severely limited by their poor buoyancy and, above all, due to their distinct tendency to absorb water, as A. Venkateswara Rao, Nagaraja D. Hegde and Hiroshi Hirashima, Absorption and desorption of organic liquids in elastic superhydrophobic silica aerogels, Journal of Colloid and Interface Science, 305(1):124-132, 2007 shows. As well as their low absorbing capacity, then, the major disadvantage of these hydrophilic absorbents is in that, as well as oil, they also absorb a considerable quantity of water. This necessitates reprocessing the absorbed liquids. Elaborate techniques such as sedimentation, floatation or centrifugation are used in sequence for this purpose. These methods, however, can only separate emulsions above a certain particle size. They can only be utilized to a limited extent for particle sizes under 150 μm. Xinwei Chen, Liang Hong, Yanfang Xu and Zheng Wei Ong, Ceramic pore channels with inducted carbon nanotubes for removing oil from water, ACS Applied Materials & Interfaces, 4 (4): 1909-1918, 2012 use complex methods for this, based on carbon nanotubes. Sorbents are required, which repel water and hence selectively absorb oil, in order to avoid a similarly elaborate processing of skimmed liquid.
These types of sorbents are mineral-based. Mineral absorbents such as zeolite or silica foams are first and foremost amphiphilic (hydrophilic and lipophilic). M. O. Adebajo, R. L. Frost, J. T. Kloprogge, O. Carmody and S. Kokot, Porous materials for oil spill cleanup: A review of synthesis and absorbing properties, Journal of Porous Materials, 10:159-170, 2003 discuss a suitable treatment, which hydrophobizes these absorbents, whereby the penetration of water into the micropores and nanopores is expected to be prevented. As is generally common for mineral absorbents, however, the following negative characteristics are in opposition to the high oil absorbing capacity: high flammability, high brittleness and a hydrophobicity which cannot be permanently maintained. Should water penetrate the foams, which cannot be prevented during operation in the sea, these absorbents collapse, as a rule, owing to the current and the swell. Other sorbents available on the market and in the research area are synthetic organic absorbents, such as commercially available polypropylene or polyurethane mats, which seem to close this gap. These are primarily characterized by a rapid oil absorption and high degree of oil absorption. The absorption of water is low, but nevertheless exists. On removing these mats from the water, however, one weakness is noticeable: the retention of the oil is severely limited. This results in approximately 50% of the oil collected flowing back into the sea on removing the mats. Thus, the bulk of the available absorbents does not fulfill the required hydrophobicity. Along with oil, water is also absorbed, at times in very large volumes. The selective absorption of oil is not sufficiently guaranteed, either with natural or with mineral absorbents. Under certain circumstances, the absorption of water can even result in the complete destruction of the absorbency due to structural collapse. Here, the organic absorbents have advantages, but their retention capacity is limited. Up to 50% of the absorbed oil is released back into the sea on the removal of the absorbents from the water.